I. Field of the Invention
The present invention relates to a wireless communication system for transmitting multiple channels of information to recipients which selectively demodulate only one channel at a time. More specifically, the invention relates to a method and apparatus for establishing or maintaining signal diversity in a satellite-based communication system in which multiple digital modulation channels are transmitted simultaneously to system users, and on the same frequency.
II. Related Art
New communication services have been proposed to provide pay-per listen types of services for mobile service users or consumers. A typical application that has been recently authorized for deployment in the United States is a consumer radio product referred to as Digital Audio Service (DAS) or Digital Audio Broadcasting (DAB). This type of service is intended for receiving one of a set of channels bearing information such as news, sports, music, weather, or other thematic entertainment presented with high audio quality. The quality level of the music is generally represented as being the same as that seen for optical storage media such as Compact (audio/optical) Discs commonly referred to as CDs, although a compression of approximately 10:1 is usually employed. These services are typically planned to operate over a bandwidth of about 12.5 MHz with a center frequency of around 2.3 GHz (referred to as the user reception frequency).
A variety of communication techniques have been developed for transferring information among or to a large number of system users at such frequencies. Techniques, such as time division multiplexing (TDM), frequency division multiplexing (FDM), and code division multiplexing (CDM) spread spectrum techniques. The use of CDM is documented for example in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters,xe2x80x9d and U.S. Pat. No. 5,691,974, entitled xe2x80x9cMethod And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy,xe2x80x9d each of which are incorporated herein by reference.
While techniques such as CDM provide a relatively high signal gain and permit problems such as multipath fading to be more readily overcome, they do not totally eliminate such problems. Therefore, it is desirable to provide some form of signal diversity to reduce the deleterious effects of fading and additional problems associated with relative user movement. In addition, signal blockage by occasional buildings, foliage (trees), or geographic features (mountains and hills) can contribute to severe attenuation or complete blockage of some signals, preventing appropriate signal reception in some situations.
The problem of signal blockage takes on added importance where a long term communication link is desired. That is, instead of relatively short messages or calls, transmissions on the order of several minutes or more are desired. For example, transfer of radio like or audio programs through satellites, or large volume dedicated data transfers, require the receiver to have an adequate signal for many minutes or hours at a time. Signal blockage sufficient to interrupt this type of link is very deleterious, and can cause economic losses and long term customer base erosion where the link is part of a fee-for-service type link.
In order to assure high quality and uninterrupted reception of signals some form of signal diversity needs to be used. Generally, three types of diversity are commonly used in communication systems, and they are time, frequency, and space diversity. Time diversity is obtainable using data repetition, and time interleaving of data or signal components. A form of frequency diversity is inherently provided by techniques such as CDM in which the signal energy is spread over a wide bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links with a mobile or remote signal recipient through two or more base stations for terrestrial systems; or two or more satellites or satellite beams, for space-based systems.
That is, for satellite communication systems, space or path diversity is obtained by transmitting, or receiving, over multiple signal paths and allowing a signal arriving over different paths to be received and processed separately for each path. Examples of using path diversity in multiple access communication systems are illustrated in U.S. Pat. Nos. 5,101,501 entitled xe2x80x9cSoft Handoff In A CDMA Cellular Telephone System,xe2x80x9d and 5,109,390 entitled xe2x80x9cDiversity Receiver In A CDAM Cellular Telephone System,xe2x80x9d which are incorporated herein by reference.
In order to provide robust signal or space diversity, satellite based communication systems or information transfer services, can transfer signals through two or more satellites at any time to an intended recipient. The signals are encoded in such a manner as to allow a user receiver to distinguish between them and process them accordingly. For TDM systems this means assigning different time slots or sets of slots, and for CDM systems this means assigning different PN code offsets or codes. Different frequencies can also be used in some systems, although far less desirable. Multiple layers or levels of coding can also be used. Generally, the user receiver then employs a series of digital receivers or what is referred to as a rake receiver, for accomplishing the signal reception.
However, typical satellite systems may have a lower than desired margin for signal separation. That is, unless certain techniques are employed, it is difficult to maintain separation of the signals due to path variations, and so forth. Unfortunately, these techniques require circuitry and software that makes receivers more expensive than desired or practical for some applications such as in the consumer products intended for use with the above audio systems. For example, when manufacturing receivers for use in pay-per-listen mobile applications on a large mass-market scale, costs and complexity are extremely important. Every small cost increment can greatly effect the profitability of an information service enterprise, and complexity can impact to both cost and reliability.
What is needed is a technique or apparatus that maintains a high quality communication link by providing or enhancing signal diversity while providing an inexpensive and reliable receiver suitable for mass produced consumer electronics.
One object of the present invention is to mitigate the effect of signal blockages from physical structures through appropriate signal diversity.
An advantage of the invention is that diversity reception can be maintained with no increase in receiver complexity. In fact, a lower cost RF front end may be accommodated while maintaining the desired high quality link.
The features and advantages of the present invention are realized by using at least two signal sources, generally satellites, and polarized radiation to transfer the desired communication signals to end users, customers, or service subscribers. Two polarization modes are employed, with the preferred modes being left-hand and right-hand circularly polarized radiation. However, in some configurations, vertical and horizontal polarized radiation may be employed. The total number of channels can be divided among the polarization modes and transferred to end users either through the use of polarization switching in the receivers to selectively switch between the satellites, or by switching elements in the satellites (sources) to switch between polarization modes used for transmission.